THE PHYSIOLOGY OF RESPIRATION IN INSECTS. 377 
The function of the abdominal muscles is to maintain the 
air-pressure, and to expel the generative products and feces. 
The alternate contraction and expansion of the abdomen which 
occur during flight are not respiratory, but a means of regulating 
the intra-thoracic air-pressure. I have frequently convinced 
myself that there are no rhythmic movements of the body wall 
in the blow-fly when at rest. 
Thoracic Respiratory Movements.—At first sight it will appear, 
perhaps, as many have held, that inspiration is effected by the 
enlargement of the thorax, and I think it possible that when 
the intra-thoracic air-pressure is low, some air may be drawn 
into the thoracic trachez by the contraction of the longitudinal 
thoracic muscles (dorsales). There is, however, no doubt in 
my mind that the air-pressure in the trachee is normally 
higher than the atmospheric pressure; the air cannot in this 
case be renewed by expansion of the thorax. 
The Intra-tracheal Pressure——That the air contained in the 
tracheal tubes is compressed, is shown by the fact that the 
membranous tracheze may frequently be seen to expand when 
relieved of the pressure of surrounding parts; and by the con- 
tinuous nature of the humming sound produced by the escape 
of air from the tympanic spiracle. I have been unable to 
determine the internal pressure, but the following facts, I think, 
justify the assumption that it is not less than two or three 
inches of mercury, higher than that of the surrounding atmo- 
sphere, in a vigorous blow-fly. 
If an opening is made into the great air vessels the insect 
becomes helpless. I believe the experiment, which has fre- 
quently been made to determine the function of the balancers 
by cutting them off, an operation which destroys the insect’s 
power of flight, or even of supporting its own weight on its 
legs, really gives rise to these phenomena by reducing the 
internal air-pressure ; as it is impossible to remove these organs 
without laying open the great tracheal trunks in the vicinity of 
the posterior spiracles. 
The proboscis is exserted and erected (see Chap. XI., Sect. 1) 
by the air-pressure in its great tracheal trunks, and, as 
